In a fuel cell system, plate-like cells each considered as one unit are stacked in a thickness direction to form what is called a cell stack as a principal component of the system. Though depending on the type of a fuel cell, the cell stack is generally formed of a reformer to generate hydrogen-rich reformed gas to be supplied to the cell stack, an evaporator to generate steam for the reforming, and a heat exchanger to preheat air or the like to be supplied as oxidized gas to the cell stack by using exhaust gas resulting from combustion of off-gas released from the cell stack, for example. These components are accommodated together in a casing to become a product as a fuel cell module. The off-gas collectively means gases to be released from the cell stack. The off-gas includes unburned gas released from the cell stack while power is generated, and city gas and reformed gas released from the cell stack at the start of operation before power is generated, for example.
The performance of such a fuel cell system has been enhanced remarkably in recent years which has facilitated size reduction of the system. Fuel cell systems for home use have started to become commercially available.
A fuel cell system for home use is required to satisfy requirements in terms of cost and size. Regarding the size, the size of a fuel cell module as the aforementioned system main body is given particular importance. This is for the reason that in the case of a fuel cell for home use, a fuel cell module as the largest module is generally given limited space for installation and should be installed in a narrow space in many cases. This necessitates further reduction in size of the fuel cell module. However, the fuel cell module cannot easily be reduced further in size than it is at present.
Specifically, a main mechanism in the fuel cell module is the cell stack. The cell stack is actually the largest part and occupies the largest space in the casing. Modifications have been made in every aspect for size reduction of the cell stack. Meanwhile, the reformer, the evaporator, etc. as auxiliary mechanisms are substantially smaller than the cell stack as the main mechanism. Modifications having been made for the auxiliary mechanisms have been limited to aspects of efficiency and durability. Auxiliary mechanisms in a two-stage cylindrical form are described as efficient mechanisms in patent literatures 1 and 2. This two-stage cylindrical form includes an evaporator formed into a vertical cylindrical shape and a reformer also formed into a vertical cylindrical shape coaxially coupled to the evaporator in a position above the evaporator. Auxiliary mechanisms in a double cylindrical form are also known (patent literature 3). This double cylindrical form includes a reformer formed into a vertical cylindrical shape and an evaporator also formed into a vertical cylindrical shape combined to the reformer outside the reformer in a concentric pattern.
The reformer and the evaporator use combustion exhaust gas as a heat source that results from combustion of off-gas such as unburned gas released from the cell stack without having been used for power generation in the cell stack. In particular, steam reforming is a reaction of absorbing heat. Thus, heating in the reformer, more specifically heat exchange with the combustion exhaust gas becomes an important issue. The two-stage cylindrical form and the double cylindrical form intended for integration of the reformer and the evaporator act advantageously from a viewpoint of usage of heat of the combustion exhaust gas.